The present invention relates to a method and a device for controlling a fuel injection system for an internal combustion engine of a motor vehicle.
The applicant knows of fuel injection systems having a high-pressure fuel storage system (hereinafter referred to as common rail), a high-pressure pump, a flow control valve for controlling the fuel delivery rate of the high-pressure pump into the common rail and a control unit for triggering the flow control valve.
German Patent DE 198 34 120 also describes a fuel injection system having a flow control valve that controls the pressure buildup in the common rail. Such flow control valves are used for direct injection internal combustion engines in particular.
German Patent DE 199 12 966 A1 describes a fuel injection system in which the pressure in the common rail is created by suitable control of the flow control valve. The flow control valve is arranged on the intake side of the high-pressure pump. As long as the flow control valve is open, the high-pressure pump can draw in fuel, increase its pressure and then deliver it into the common rail.
In the above approaches, however, the fact that the flow control valve is triggered as a function of the pump lift of the high-pressure pump, i.e., in synchronization with the pump lift, has proven to be a disadvantage. In the case of a single-plunger high-pressure pump which is used to generate a fuel high pressure with direct gasoline injection and is driven directly by the internal combustion engine, the high-pressure pump is driven via a camshaft having one or more control cams. The maximum pump delivery is increased with an increase in the diameter and lift of the plunger of the single-plunger high-pressure pump and with an increase in the number of control cams on the camshaft. In general, two control cams are used per cam revolution on the camshaft or, for a higher fuel delivery rate, three control cams may be used. For uniform triggering of the flow control valve in synchronization with the pump lift according to the known principles given above, however, the flow control valve must be triggered in the correct phase with each cam lift because control of the fuel delivery is accomplished by defining the point in time of triggering of the flow control valve.
With these approaches, the disadvantage is that the number of control cams on the camshaft and thus the maximum delivery rate are limited, especially at high engine speeds, because of excessively high triggering frequencies.
To increase the maximum fuel delivery rate and be able to ensure triggering in the correct phase even at high engine speeds, the present applicant is aware of the approach in which two parallel high-pressure pumps each having two control cams are provided instead of a high-pressure pump triggered via a camshaft having four control cams.
However, it has proven to be a disadvantage of this approach that a parallel arrangement of two or more high-pressure pumps results in an increased need for space, increased control complexity, increased manufacturing costs and increased manufacturing complexity.
The object of the present invention is thus to create an improved method and an improved device for controlling a fuel injection system which will ensure accurate control of the desired fuel delivery rate even at high engine speeds.
The idea on which the present invention is based is to provide a method and a device for controlling a fuel injection system for an internal combustion engine of a motor vehicle, whereby the internal combustion engine has a common rail, a high-pressure pump, a flow control valve for controlling the fuel delivery rate of the high-pressure pump into the common rail and a control unit for controlling the flow control valve, whereby the prevailing fuel demand in the common rail is determined as a function of the prevailing driving state of the vehicle by means of a detection device, and the pulse duty factor of the triggering of the flow control valve is controlled by the control unit as a function of the prevailing fuel demand thus determined in the common rail by means of the control unit, whereby the control is executed in synchronization with and independently of the pump lift of the high-pressure pump.
The present invention thus has the advantage over the known approaches that control of the pulse duty factor of the triggering of the flow control valve is implemented not in synchronization with the pump lift but instead in synchronization with the prevailing fuel demand as just determined. The frequency for triggering the flow control valve can thus be separated from the pump lift frequency and reduced, if necessary, and a high-pressure pump can also be triggered by a camshaft having four control cams, for example, even at high rotational speeds. The maximum delivery rate can also be increased to advantage by using four control cams, for example, without resulting in any synchronization problems.
According to a preferred embodiment, the fuel pressure in the common rail is detected by a pressure sensor, whereby the prevailing fuel demand in the common rail is determined by comparing the fuel pressure detected with a setpoint pressure. For example, corresponding setpoint pressures are stored in an engine characteristics map in a respective memory device. Thus by comparing the setpoint pressure with the actual pressure, the prevailing fuel demand in the common rail is detected by a simple method, so that the control of the pulse duty factor of the triggering of the flow control valve can be adapted to the fuel demand detected for supplying a suitable fuel delivery rate to the common rail.
According to another preferred embodiment, the internal combustion engine is designed as a four-cycle engine of a motor vehicle, whereby the high-pressure pump is designed for executing multiple delivery phases during an operating cycle of a cylinder of the internal combustion engine. The high-pressure pump is preferably designed as a single-plunger high-pressure pump. The working cycle of an internal combustion engine that operates by the four-cycle method is understood to be a crank angle of 720° in conjunction with the present invention because the four cycles of an internal combustion engine—intake, compression, power and exhaust take place within this crank angle.
According to another preferred embodiment, the pulse duty factor of the triggering of the flow control valve is controlled as a function of the fuel pressure detected in the common rail and as a function of the rotational speed of the internal combustion engine. In this way, the pulse duty factor of the triggering of the flow control valve can be controlled suitably as a function of the prevailing driving state of the vehicle, whereby both the prevailing engine speed and the prevailing engine load are advantageously taken into account in triggering the pulse duty factor.
Up to a predetermined limit rotational speed of the internal combustion engine, four delivery strokes of the high-pressure pump are preferably preselected, where the flow control valve is triggered uniformly with a predetermined basic frequency as a function of the pump lift, i.e., with pump lift synchronization. Therefore, the flow control valve is triggered with a trigger frequency in the noncritical engine speed range up to the limit speed. Only beyond the predetermined limit speed of the internal combustion engine is the pulse duty factor of the triggering of the flow control valve advantageously controlled as a function of the prevailing fuel demand in the common rail as thus determined. The pulse duty factor is thus controlled in synchronization not with the pump lift but instead with time and as a function of the instantaneous engine load. Thus, at a critical engine speed, i.e., beyond the predetermined limit speed, the trigger frequency of the flow control valve, for example, can be reduced in a suitable manner without falling below the fuel delivery rate required at that moment.
According to another preferred exemplary embodiment, the frequency for triggering the flow control valve is reduced by one half in comparison with a predetermined basic frequency beyond the predetermined limit speed of the internal combustion engine, such that the flow control valve is triggered with every second delivery stroke of the high-pressure pump. In this way, even at high engine speeds, a sufficient amount of time is available for accurate triggering of the flow control valve and satisfactory triggering is ensured. In the case of a high fuel demand in the common rail, the pulse duty factor of triggering the flow control valve is preferably controlled beyond the predetermined limit speed of the internal combustion engine in such a way that the flow control valve is triggered with every second delivery stroke of the high-pressure pump for a full delivery stroke and with the in-between delivery strokes of the high-pressure pump, it is triggered for regulating strokes. As an alternative, when there is a low fuel demand in the common rail, the pulse duty factor of triggering of the flow control valve is preferably controlled beyond the predetermined limit speed of the internal combustion engine such that the flow control valve is triggered for a zero delivery stroke with every second delivery stroke of the high-pressure pump and for regulating strokes with the in-between delivery strokes of the high-pressure pump.
In the case of an intermediate fuel demand between the high and low fuel demand in the common rail, beyond the predetermined limit speed of the internal combustion engine, the pulse duty factor of the triggering of the flow control valve is preferably controlled by a mixed control which consists of alternating operation of the two alternative control operations described above. High-speed operation may thus be accomplished even with a single high-pressure pump that is triggered by a camshaft having four triggering cams, i.e., the usable speed range is advantageously expanded.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
Unless otherwise indicated, the same reference notation is used to refer to the same or functionally identical components in the figures.